The present invention relates to a synthetic quartz glass for optical components to be used for an apparatus employing ultraviolet lights having wavelengths of from 150 to 200 nm as a light source, and a process for producing it, particularly to a synthetic quartz glass to be used as optical components such as a lens, a prism, a photomask, a pellicle and a material for windows, to be used for light within a range of from the vacuum ultraviolet region and the ultraviolet region, such as an ArF excimer laser (wavelength: 193 nm), a F2 laser (wavelength: 157 nm), a low pressure mercury lamp (wavelength: 185 nm) or a Xe2★ excimer lamp (wavelength: 172 nm).
A synthetic quartz glass has such characteristics that it is a transparent material within a wavelength range of as wide as from the near infrared region to the ultraviolet region, it has an extremely small thermal expansion coefficient and is excellent in dimensional stability, and it contains substantially no metal impurity and has a high purity. Accordingly, a synthetic quartz glass has been mainly used for optical components of a conventional optical apparatus employing g-line or i-line as a light source.
Along with high-integration of LSI in recent years, techniques to draw finer and thinner lines have been required in an optical lithography technology to draw an integration circuit pattern on a wafer, and accordingly use of light having a shorter wavelength as an exposure light source has been promoted. For example, for a light source of a stepper for optical lithography, an ArF excimer laser (hereinafter referred to simply as an ArF laser) or a F2 laser is now to be used, as advanced from conventional g-line (wavelength: 436 nm) and i-line (wavelength: 365 nm).
Further, a low pressure mercury lamp or a Xe2★ excimer lamp is used for an apparatus such as optical CVD, an apparatus for cleaning silicon wafers or an ozone-generation apparatus, and it is being developed to apply it to the optical lithography technology in future.
It is necessary to use a synthetic quartz glass for a gas filled tube of a lamp to be used for a low pressure mercury lamp or an excimer lamp, or an optical element to be used for an optical apparatus employing the above-mentioned short wavelength light source.
A synthetic quartz glass to be used for such optical systems, is required to have light transmittance within a wavelength range of from the ultraviolet region to the vacuum ultraviolet region, and it is required that the light transmittance at the service wavelength will not decrease after irradiation of light.
With a conventional synthetic quartz glass, if it is irradiated with light from a high energy light source such as an ArF laser or a F2 laser, a new absorption band will be formed in a ultraviolet region, and such has been problematic for an optical component to be used for constituting an optical system employing an ArF laser or a F2 laser as a light source.
If an ArF laser or a F2 laser is applied for a long time, an absorption band (hereinafter referred to as a 214 nm absorption band) having a wavelength of 214 nm at the center, so-called an Exe2x80x2 center (xe2x89xa1Si.), and an absorption band (hereinafter referred to as a 260 nm absorption band) having a wavelength of 260 nm at the center, so-called NBOHC (non-crosslinked oxygen radical: xe2x89xa1Sixe2x80x94O.), will be formed.
As a technique to suppress formation of such absorption bands, a method of incorporating at least 100 ppm of OH groups and at least 5xc3x971016 molecules/cm3 of hydrogen molecules into a synthetic quartz glass which contains substantially no reduction type defects or oxidation type defects, has been proposed (JP-A-3-101282). It is disclosed that hydrogen molecules in the synthetic quartz glass have a function to mend defects such as Exe2x80x2 centers or NBOHC formed by the ultraviolet ray irradiation, and the OH groups have a function to reduce the concentration of defect precursors which become Exe2x80x2 centers or NBOHC when irradiated with ultraviolet rays.
However, as a result of a detailed research on the change in light transmittance of a synthetic quartz glass by ultraviolet ray irradiation, the present inventors have found that in the synthetic quartz glass, an absorption band (hereinafter referred to as a 163 nm absorption band) having a wavelength of 163 nm at the center, will be formed in addition to the 214 nm absorption band and the 260 nm absorption band. When it is used as an optical component for an apparatus employing light with a wavelength of at least 200 nm as a light source, the service wavelength and the 163 nm absorption band are apart, whereby there will be no substantial influence of the decrease in light transmittance due to formation of the 163 nm absorption band. However, in a case where it is used as an optical component for an apparatus employing light with a wavelength of from 150 to 200 nm as a light source, the light transmittance in the vicinity of the service wavelength will decrease due to formation of the 163 nm absorption band.
The present invention has an object to provide a synthetic quartz glass which is to be used for an apparatus employing ultraviolet rays having wavelengths of from 150 to 200 nm as a light source and which has high light transmittance at a wavelength of from 150 to 200 nm and is excellent in ultraviolet ray resistance (the light transmittance in the vicinity of the service wavelength will not decrease even when irradiated with light employing ultraviolet rays with wavelengths of from 150 to 200 nm as a light source), and a process for its production.
The present invention provides a synthetic quartz glass to be used for light with a wavelength of from 150 to 200 nm, wherein the OH group concentration in the synthetic quartz glass is at most 100 ppm, the hydrogen molecule concentration is at most 1xc3x971017 molecules/cm3, reduction type defects are at most 1xc3x971015 defects/cm3, and the relation between the change xcex94k163 in the absorption coefficient at a wavelength of 163 nm and the change xcex94k190 in the absorption coefficient at a wavelength of 190 nm, as between before and after irradiation of ultraviolet rays with a wavelength of at most 250 nm, satisfies 0 less than xcex94k163 less than xcex94k190.
It is important that the OH group concentration is at most 100 ppm (meant for weight ppm), and in a case where it is used as an optical component for an apparatus employing light in a vacuum ultraviolet region having a wavelength of at most 180 nm as a light source, the OH group concentration is preferably at most 50 ppm, particularly preferably at most 10 ppm. The lower the OH group concentration, the higher the light transmittance.
When a synthetic quartz glass containing hydrogen molecules is irradiated with ultraviolet rays, the 163 nm absorption band will be formed. The 163 nm absorption band is attributable to reduction type defects (xe2x89xa1Sixe2x80x94Sixe2x89xa1 bonds) and will substantially lower the light transmittance of ultraviolet rays with wavelengths of at most 200 nm. With a view to suppressing formation of the 163 nm absorption band, it is important that the hydrogen molecule concentration in the synthetic quartz glass is at most 1xc3x971017 molecules/cm3, particularly preferably at most 3xc3x971016 molecules/cm3.
To accomplish the object of the present invention, it is necessary to suppress formation of the 163 nm absorption band. The degree for suppression of formation of the 163 nm absorption band can be evaluated from the relation between the change xcex94k163 in the absorption coefficient at a wavelength of 163 nm and the change xcex94k190 in the absorption coefficient at a wavelength of 190 nm, as between before and after irradiation of ultraviolet rays with a wavelength of at most 250 nm. Namely, in the present invention, it is important that the relation of 0 less than xcex94k163 less than xcex94k190 is satisfied.
Further, when irradiated with ultraviolet rays, reduction type defects (xe2x89xa1Sixe2x80x94Sixe2x89xa1 bonds) in the synthetic quartz glass permit formation of the 214 nm absorption band (xe2x89xa1Si.) by the formula xe2x89xa1Sixe2x80x94Sixe2x89xa1+hxcexdxe2x86x92xe2x89xa1Si.+xe2x89xa1Si. thereby to lower the light transmittance of ultraviolet rays at a wavelength of from 150 to 200 nm. Accordingly, in the present invention, it is important that the reduction type defects in the synthetic quartz glass are at most 1xc3x971015 defects/cm3. The concentration of the reduction type defects can be obtained from the absorption intensity at a wavelength of 163 nm (Phys. Rev., B38, 12772 (1988)).
Further, when irradiated with ultraviolet rays, the oxidation type defects (xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Sixe2x89xa1 bonds) in the synthetic quartz glass permit formation of the 260 nm absorption band (xe2x89xa1Sixe2x80x94O.) by the formula xe2x89xa1Sixe2x80x94Oxe2x80x94Oxe2x80x94Sixe2x89xa1+hxcexdxe2x86x92xe2x89xa1Sixe2x80x94O.+xe2x89xa1Sixe2x80x94O., whereby the light transmittance of ultraviolet rays at a wavelength of from 150 to 200 nm will be adversely affected depending upon the degree of the formation. Accordingly, in the present invention, it is preferred that the oxidation type defects in the synthetic quartz glass are at most 2xc3x971017 defects/cm3. The concentration of the oxidation type defects can be obtained from the OH group concentration increased by heat treatment of the synthetic quartz glass (10 mm in thickness) in 100% hydrogen gas at 1 atm at 900xc2x0 C. for 24 hours.
Further, in the present invention, chlorine element and metal impurities (such as alkali metals, alkaline earth metals, transition metals, etc.) in the synthetic quartz glass not only reduce the initial light transmittance in a wavelength range of from the vacuum ultraviolet region to the ultraviolet region but also cause to reduce the ultraviolet ray resistance. Accordingly, the smaller their contents, the better. The content of the metal impurities is preferably at most 100 ppb (meant for weight ppb), particularly preferably at most 10 ppb. The concentration of the chlorine element is preferably at most 100 ppm (meant for weight ppm), particularly preferably at most 10 ppm, further preferably at most 2 ppm.
The synthetic quartz glass of the present invention can be prepared by a direct method, a soot method (VAD method, OVD method) or a plasma method. Particularly preferred is a soot method, whereby control of the OH group concentration in the synthetic quartz glass is relatively easy, and the temperature of the preparation is low, which is advantageous with a view to avoiding inclusion of impurities such as chlorine and metals.
Further, the present invention provides a process for producing a synthetic quartz glass, which comprises carrying out in this order:
(1) a step of depositing and growing, on a substrate, fine particles of quartz glass obtained by subjecting a glass-forming material to flame hydrolysis in an oxidizing atmosphere, to form a porous quartz glass body;
(2) a step of heating the porous quartz glass body at a temperature of at least 1,400xc2x0 C. to obtain a transparent glass body;
(3) a step of heating the transparent glass body in an atmosphere containing hydrogen to dope it with hydrogen; and
(4) a step of heating the transparent glass body in an atmosphere containing no hydrogen for dehydrogenation treatment to obtain the above described synthetic quartz glass.
The glass-forming material may, for example, be a halogenated silicon compound (for example, a halide such as SiCl4, SiHCl3, SiH2Cl2 or CH3SiCl3, a fluoride such as SiF4, SiHF3 or SiH2F2, a bromide such as SiBr4 or SiHBr3, or an iodide such as SiI4) or an alkoxysilane (for example, RnSi(OR)4xe2x88x92n (wherein R is a C1-4 alkyl group, and n is an integer of from 0 to 3)). With a view to reducing inclusion of impurities such as metals and chlorine, an alkoxysilane (particularly methyl trimethoxysilane, tetramethoxysilane, etc.) is preferred. Further, from the viewpoint of the operation efficiency or cost, SiCl4 or the like is preferably employed.
As the substrate, a seed rod made of quartz glass can be used. Further, not only a rod shape, but also a plate-shaped substrate may be employed.
In the present invention, it is preferred to include, between steps (1) and (2), a step (1a) of heating the porous quartz glass body at a temperature of from 900 to 13000C to remove water, with a view to adjusting the OH group concentration to at most 100 ppm. The atmosphere in this step (1a) is preferably an atmosphere comprising an inert gas such as helium as the main component (inclusive of a case where the inert gas is 100%). The pressure (the absolute pressure, the same applies hereinafter) is preferably a reduced pressure or atmospheric pressure, particularly preferably at most 100 Torr (1.33xc3x97104 Pa), more preferably at most 10 Torr (1.33xc3x97103 Pa).
Further, in the present invention, it is preferred to include, between steps (1) and (2), a step (1b) of exposing the porous quartz glass body to an atmosphere containing a fluorine-containing gas to dope it with fluorine, with a view to adjusting the OH group concentration to at most 100 ppm. In such a case, fluorine will be contained in the synthetic quartz glass in an amount of from 100 to 10000 ppm, preferably from 1000 to 8000 ppm, particularly preferably from 3000 to 8000 ppm. As the fluorine-containing gas, SiF4, SF6, CHF3, CF4 or F2 may, for example, be mentioned. The atmosphere containing the fluorine-containing gas, is preferably an inert gas containing from 0.1 to 100 vol %, particularly from 0.1 to 25 vol %, of the fluorine-containing gas. In such a case, the temperature is preferably at most 600xc2x0 C. In a case where fluorine is doped at a high temperature of from 500 to 1150xc2x0 C., it is preferred to suppress formation of reduction type defects by using an atmosphere containing the fluorine-containing gas in an amount of from 0.1 to 100 vol %, particularly from 0.1 to 25 vol %, and further oxygen in an amount of from 5 to 95 vol %, particularly from 50 to 95 vol %. The pressure at the time of fluorine doping, is preferably from 0.1 to 10 atm (from 1.013xc3x97104 to 1.013xc3x97106 Pa). Further, the time is preferably from a few hours to a few tens hours.
In the present invention, when both steps of steps (1a) and (1b) are to be carried out, it is preferred to carry out step (1b) prior to step (1a).
In the present invention, it is preferred that the heating temperature in step (3) is at most 600xc2x0 C. with a view to suppressing formation of reduction type defects.
Further, in the present invention, it is preferred that the heating temperature in step (4) is at most 600xc2x0 C. with a view to suppressing formation of reduction type defects.